Retrieves the content at the given URL as a string.
| url | 
                 The URL for the content to get as a string  | 
            
| cancellable | 
                 a GLib.Cancellable  | 
            
| 
                 the content at the URL as a string  | 
            
| GLib.Error | 
                 if an error occurs while downloading the content  |